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Every Union monster has an Ignition Effect that allows it to either equip to or unequip from the host once per turn while it is on the field. If a Union monster unequips itself with its own effect, it is Special Summoned; Union monsters last printed prior to Structure Deck: Seto Kaiba can only be Special Summoned in Attack Position if Summoned this way. Additional cards, such as "Formation Union" and "Combination Attack", can equip and unequip Union monsters, with "Formation Union" even treating the Union monster as having been equipped by its own effect.

They also do not require you to Special Summon them in Attack Position after being unequipped from a monster.

Older Union monsters have an effect to destroy themselves if the monster they are equipped to would be destroyed by battle. More recent Union monsters destroy themselves if the monster they are equipped to would be destroyed by any means. All Union monsters printed since Structure Deck: Seto Kaiba (including reprints) destroy themselves if the monster they are equipped to would be destroyed by battle or card effect.

Prior to Structure Deck: Seto Kaiba, all Union monsters had a condition that meant that only one Union monster could be equipped to a monster at a time. In the OCG, from Structure Deck: Seto Kaiba onward, Union monsters no longer have this restriction (including reprints).

Once per turn, you can either: Target 1 [X] monster you control; equip this card to that target, OR: Unequip this card and Special Summon it. A monster equipped with this card [...], also if the equipped monster would be destroyed by battle or card effect, destroy this card instead.